Boston College

GPS Positioning Errors in Solar Cycle 24

William McNeil, Keith Groves and Charles Carrano

william.mcneil@bc.edu

14th International Ionospheric Effects Symposium 12-14 May 2015

Outline

• Solar cycle review • Case studies of positioning

errors • Latitude dependence of

positioning errors • Position errors during past

and present solar cycles • Summary

Birth of Solar Cycle 24 Jan 4, 2008

STEREO EUV Image NASA

NOAA Solar Cycle Data & Projection

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Monthly Peaks around 100

Solar Cycle 24 is somewhat less active than Solar Cycle 23, but note the peaks in late 2011 and early 2014.

Cuiaba, Brazil Solar Cycle Variations in GPS Scintillations

2011

2013

2012

2011

2010

2009

2008

2007

2006

2005

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Total Electron Content Variations with Solar Cycle

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GPS scintillation turns on when TEC exceeds ~50 TECU

A Study of GPS Positioning Errors Seven stations at varying magnetic latitudes

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Determining “Truth” Positions

• We find the “truth” location from a 24-hour quiet period. • Different receivers give different noise levels, but all noise

levels are small compared to scintillation-induced errors.

Examples from March 2012

• Considering specific days we see direct correlation between scintillation and horizontal position errors.

• The scintillation-induced errors on these nights are small.

Examples of Larger Errors

• We have found several days with errors of several 10’s of meters in both the horizontal and the altitude.

• Some of these come as a constant offset and some more as random fluctuations.

Error Statistics

• We generated histograms of errors from the seven stations to examine the effect of magnetic latitude.

• The following slides show the progression as we go from the equator toward the south pole.

Sao Luis – 0.1° MLat

Cuiaba – 6.5° MLat

Ilheus – 15.2° MLat

Ascension Island – 17.8° MLat

Cachoeira Paulista – 19.6° MLat

Santa Fe – 22.2° MLat

Villegas – 23.6° MLat

Magnetic Latitude Dependence

Station Magnetic Latitude

Error Level

Sao Luis 0.1ᵒ None Cuiaba 6.5ᵒ Weak Ilheus 15.2ᵒ Strong

Ascension Island 17.8ᵒ Strong Cachoeira Paulista 19.6ᵒ Strong

Santa Fe 22.2ᵒ Weak Villegas 23.6ᵒ None

• Qualitatively speaking, the errors are strongest under the anomaly crest, going to zero equatorward and poleward.

• The errors are limited to a band from about 7ᵒ to about 22ᵒ magnetic latitude.

Position Errors at Ascension Island Comparison with Last Solar Cycle

• Here we compare Fall of 2011 to Spring of 2002. • We see that the errors are comparable for comparable

monthly sunspot numbers. • Here we are comparing two different receivers.

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Summary

• Ionospheric scintillation has recently entered the solar maximum phase of cycle 24

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• Users should expect impacts on GPS systems through 2015

• We find errors up to 100 meters during periods of intense scintillation

• The errors are confined to a relatively broad band in magnetic latitude corresponding roughly to the latitudes of the equatorial anomaly

• GPS positioning equatorward of about 7ᵒ and poleward of about 22ᵒ is not impacted by equatorial scintillation..

SCINDA Sites Map

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Existing and expected sites through 2012

How Important are These Errors?

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The impacts of errors of this magnitude will of course depend on the application, but as a reference, we quote Seo et al. [2010] “The alert limits of the LPV-200 approach procedure are 40 meters in the horizontal and 35 meters in altitude. Seo, J., T. Walter and P. Enge, Correlation of GPS signal fades due to ionospheric scintillation for aviation applications, Adv. Space Res, 47, 1777 (2010)

Local Time Dependence

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Existing and expected sites through 2012

Solar Cycle Dependence

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Relative Occurrence of Bubbles Exceeding 1000 km Altitude

• Sites at different latitudes see different levels of activity depending on bubble altitude

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